Journal of Metals, Materials and Minerals, Vol.23 No.1 pp.29-33, 2013
Conversion of Secondary Pulp and Paper Sludge to Glucose
by Hydrothermal Treatment
Nawapon CHARIYAPRASERTSIN1, Yukihiko MATSUMURA2
and Tawatchai CHARINPANITKUL1,*
1
Center of Excellence in Particle Technology, Department of Chemical Engineering,
Faculty of Engineering, Chulalongkorn University, Bangkok, 10330, THAILAND
2
Department of Mechanical Systems Engineering, Hiroshima University,
1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527 Japan
Abstract
Secondary pulp/paper sludge is residue from water treatment process of the paper industry. Main
components of secondary pulp/paper sludge are cellulose, hemicelluose, lignin and calcium carbonate
(CaCO3). The cellulose and hemicellulose could be converted to glucose by hydrothermal treatment in a
stainless steel tubular batch reactor. It was found that dried powder of secondary pulp/paper sludge with
nominal size in a range of 595 to 840 µm contains different content of CaCO3. Therefore, effect of CaCO3
content on glucose yields was examined under designated temperature of 200-260 ºC and treating time of 040 minutes. Identification and quantification of glucose and other derivatives in aqueous phase was
performed with high-performance liquid chromatography. Plausible reaction pathway of cellulose subject to
hydrothermal treatment with the presence of CaCO3 was proposed and discussed. The highest glucose yield
could be obtained from secondary sludge at 220oC with 20 min holding time. It was found that CaCO3 must
be separated before hydrothermal treatment to again higher cellulose conversion.
Key words : Hydrothermal Treatment, Pulp/paper sludge, Glucose
Introduction
In many countries, the consumption of
paper is still increasing annually, resulting in an
increase in amount of waste of many forms
including pulp/paper sludge. There are two kinds
of sludge,namely primary and secondary sludge.(1)
The primary pulp/paper sludge mainly consists of
fibers, fine fiber and chemcial fillers. Meanwhile,
the secondary pulp/paper sludge (secondary sludge,
in short) is waste generated from biological
treatment subject to aerobic digestion. In general,
the secondary sludge is a liquid suspension with high
solid content. It contains biodegradable organic
compounds and inorganic compounds including a
certain amount of heavy metals. The estimated
amount of secondary sludge in 1999 in Japan was
much more than 1.7 Mt/y . In North America, it is
reported that 4 kg of secondary sludge is produced
from 100 kg of paper. In general such sludge could
be disposed by landfilling or incinerating with
some disadvantages, such as high operating cost
because of requirement of dewatering or energy
loss due to evaporation in prior to its feeding into
incineration process.
In Thailand, an estimated amount of
secondary sludge is around 0.95 million tones/y
with an increasing trend because of an increase in
paper consumption per capita. It is reported that
the sludge disposal/ management costs would go up
to 60% of the total wastewater treatment operating
costs.(2) Nevertheless, such secondary sludge can
also be considered as waste biomass which could
be subject to biological or thermo-chemical treatment
processes for producing other useful products, such as
hydrogen or ethalnol. Utilization of such secondary
sludge would help decrease the fossil fuel consumption
which in turn reduce the greenhouse gas emission.(3,4)
In general, hydrothermal processes are
operated at relatively mild conditions without
usage of auxilarly chemicals. Such processes are
free from costly treatment. Hydrothermal treatment
of sludge in one-step processes has been of interest
for many researcher teams. For instance, Torii et
al. investigated a hydrothermal treatment process
of paper sludge in a tube bomb reactor to
hydrolyze cellulose to glucose.(5) Zhang et al.
investigated of hydrothermal on secondary sludge
and sewage sludge to convert it to biooil.(6)
Corresponding authoer E-mail: [email protected]; Tel/Fax: +66 2218 6480
CHARIYAPRASERTSIN, N. et al.
30
In this work, a one-step hydrothermal
treatment of secondary sludge was examined with
a purpose for converting cellulose and
hemicellulose in secondary sludge to glucose.
Effect of calcium carbonate on the cellulose
conversion was systematically examined and
discussed.
Materials and Experimental Procedures
Material
Secondary sludge was supplied Norske
Skog Thailand Co. Ltd., Singburi, Thailand. Major
components of secondary sludge were analyzed
based on the United States Department of Agriculture
method (TAPPI’s method) and summarized in dried
weight basis in Table 1. Figure 1. shows SEM
micrograph of typical sample of pulp and secondary
sludge which were subject to hydrothermal
treatment under the same condition.
Table 1. Compositions of screened pulp and secondary
pulp and paper sludge (>840 µm, 595-840 µm,
<595 µm) by TAPPI
Pulp
Sludge
Cellulose
0.48
>840
0.40
595-840
0.33
595
0.20
Hemicellulose
0.22
0.23
0.25
0.27
Lignin
0.18
0.18
0.20
0.19
CaCO3
0.07
0.17
0.21
0.32
P
S1
S2
S3
Experimental Method
Hydrothermal treatment was carried out in
a SUS-316 stainless steel tube reactor (Swagelok Co.)
with 1-inch O.D. and effective internal volume of
100 cm3 illustrated schematically in Figure 2. It
was heated up to targeted temperature using an
electric furnace connected with a digital controller
to detect the temperature every 1 minute. Treating
temperature was controlled in a range of 200 260ºC with holding time ranged from 10 to 4
0 min.(5) Table 2 summarizes the composition of
each secondary sludge feedstock which was
classified by screening process. S1 powder sample
is secondary sludge with a nominal size larger than
840 m, S2 with nominal size in a range of 595 to
840 m and S3 with nominal size smaller than
595 m. Pulp was a standard pulp sample taken
from the pulp production process of the same
supplier.
Table 2. Composition of each secondary sludge employed
for hydrothermal treatment
Cellulose
Pulp
S1
S2
S3
Total
DI
Initial water
Mass (g) , (g)
Composition (g)
Raw
Material
Hemicellulose Lignin CaCO3
0.66
0.66
0.66
0.66
0.32
0.38
0.50
0.89
0.27
0.30
0.40
0.63
0.08
0.30
0.42
1.06
1.35
1.65
2.00
3.25
80
80
80
80
Figure 2. Schmatic diagram of tubular reactor
Analytical method
Figure 1. SEM micrographs of (a) pulp with 100-time
magnification (b) Pulp with 1000-time
magnification, (c) secondary sludge with 100
time magnification and (d) secondary sludge
with 1000 time magnification
Identification and quantification of organic
compounds and liquid products in the aqueous
phase were performed using a high-performance
liquid chromatography (HPLC, Varian, Prostar)
Conversion of Secondary Pulp and Paper Sludge to Glucose by Hydrothermal Treatment
31
equipped with 250x4.6 mm carbonyl column with
a reflextive index (RI) detector. 69% acetonitrile:
31% H2O was used as mobile phase for detecting
saccharide content. Standardized sample size was
50 µl. The amount of each saccharide was determined
from calibration curves obtained by analyzing
standard saccharide solution with known content.
The saccharide yields were calculated by the
following equation :
Gramof saccharide
inliquidproduct
Eachsaccharide
yields(%)100(
)
Gramof cellulose
infeedstock
a
(1)
Results and Discussion
Effect of Treating Temperature
Effect of treating temperature in a range of
200-260ºC on production yields of glucose and
fructose is shown in Figure 3. It was found that an
increase in the treating temperature resulted in a
gradual decrease in glucose yiled (Figure 3(a)).
However, Figure 3(b) shows that the prodcution
yield of fructose became higher with the treating
temperature increased from 200 to 240ºC. A further
increase in the treating temperature to 240ºC would
result in a lower yield of fructose. Regarding to
characteristics of each classified secondary sludge,
similar tendency was observed. These results
would be ascribed to the competitive rate of
conversion of cellulose to polysaccharides againts
decomposition of polysacharides. Jing and Lu as
well as Khajavi et al. reported that glucose would be
decomposed by thermal treatment of cellulose in a
temperature range of 180-220ºC and 180-260ºC
respectively.(7,8) The decomposition rate was
increased with the higher reaction temperature.
Similar trend was also observed with respect to
every saccharide product. However, within the
reaction temperature range of 200-220ºC, amount
of fructose converted from cellulose was comparatively
faster than its decomposition, leading to the
increasing yield of fructose. In addition, there is
also possibility that glucose would be transformed
to fructose. Further investigation on stability of
glucose and fructose under the hydrothermal
treatment would be required for this confirmation.
However, it would be reasonable to imply that
treating temperature would exert significant effect
on the conversion of cellulose in secondary sludge
to saccharides regardless of the classified size in a
range of 595 to 840 m.
b
Figure 3. Effect of treating temperature on production
yield of glucose and fructose obtained from
hydrothermal treatment of different feedstock;
Pulp,
Sludge S1,
Sludge S2,
Sludge S3
Effect of Holding Time
Figure 4 shows the dependence of glucose
and fructose yield on the holding time when each
feedstock was kept at the targeted temperature. It
could be clearly observed that with a holding time
shorter than 20 min, the maximal yield of glucose
and fructose could be obtained. A decrease in
production yield of every monosaccharide with a
longer holding time would be attributed to the
higher decomposition of polysaccharides to other
derivatives with smaller molecular size. Sasaki et
al. reported that glucose decomposition rate which
is faster than the hydrolysis rate of cellulose would
become dominant with longer contacting time.(9)
Therefore, hydrolyzed products would be converted
or decomposed to other small compounds.
Nevertheless, it should also be noted that for S3
feedstock production yield of all saccharides was
significantly lower than any other cases. Without
holding time, S3 feedstock could not provide yield
both glucose and fructose. Theses results would be
ascribed to the presence of calcium carbonate in
the feedstock, which would be discussed further in
the next section.
32
CHARIYAPRASERTSIN, N. et al.
previous report of Torii et al. focusing on
conversion of pulp fibers to glucose, which was
significantly suppressed by the presence of calcium
carbonate added into the pulp suspension in form
or Kaolin or Talc.(5)
2H2O
H3O+ + OH(2)
CaCO3
Ca2+ + CO3 2(3)
a
CO3 2- + H3O+
HCO3 - + H2O
(4)
CaCO3 + H3O+
Ca(OH)2 + CO2 + H2O
(5)
b
Figure 4. Effect of holding time on production yield of
glucose and fructose converted from different
feedstock;
Pulp,
Sludge S1,
Sludge S2,
Sludge S3
Effect of Calcium Carbonate (CaCO3)
The presence of calcium carbonate in
secondary sludge would be anticipated to exert an
affect on the conversion of cellulose by hydrothermal
treatment because it would be dissociated to provide
alkaline ions. Figure 5. reveals the dependence of
glucose yield on the type of secondary sludge in
comparison with the pulp feedstock. It could be
clearly observed that secondary sludge which
contained different amount of calcium carbonate
would provide significantly lower yield of glucose.
The higher the content of calcium carbonate, the
lower the glucose yield. In general, hydrolysis
reaction involves with H+ which would break
glycosidic bond in cellulose chain.10) Ragarding to
reactions (2) to (5), calcium carbonate react with
water, resulting in formation of CO32-, HCO3-.
Therefore, it is reasonably anticipated that those
anions generated from calcium carbonate would
react with proton ion, resulting in hindrance of
hydrolysis of cellulose. As a result, formation of
saccharide products would be obstructed due to the
decrease in H+ ions consumed by those alkaline
anions. This result is in good agreemnet with the
Figure 5. Dependence of glucose yield on type of
secomndary sludge subject to hydrothermal
treatment with different holding time :
0 min,
10 min,
20 min,
30 min,
40 min
Conclusion
Effect of treating temperature, holding
time and calcium carbonate content on saccharide
yield produced from hydrothermal treatment of
secondary sludge was experimentally examined in
a stainless tube reactor. Hydrolysis of cellulose
is strongly dependent on the treating temperature
and holding time. However, decomposition of
saccharides generated from hydrothermal treatment
of secondary sludge would also be enhanced by the
increased temperature, resulting in a significant
decrease in saccharides yield. It was also experimentally
found that an optimal holding time of shorter than
20 min would provide the best saccharide production
yield. Calcium carbonate would probably exhibit
inhibiting effect on hydrothermal treatment of
secondary sludge, suggesting that separation of
calcium carbonate from the sludge should be taken
into account before hydrothermal treatment of
secondary sludge. The highest glucose yield obtained
from hydrothermal treatment of secondary sludge
Conversion of Secondary Pulp and Paper Sludge to Glucose by Hydrothermal Treatment
could be achieved at treating temperature of 220ºC
for 20 min holding time.
Acknowledgements
The authors gratefully acknowledge supports
of the Center of Excellence in Particle Technology,
Chulalongkorn University and the Department of
Mechanical Systems Engineering, Hiroshima.
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